Conventional audio power amplifiers suffer from low efficiency, and this causes these devices to generate heat that must be removed by large heat sinks and thereby causes the physical amplifiers to be quite large. Recently, in order to make amplifiers smaller, high-efficiency designs have been introduced. The most common solution is to use switching amplifiers, or known as class-D amplifiers. The class-D amplifiers are two to five times more efficient than class-AB amplifiers and have benefits because of the efficiency, for example, requiring smaller power supplies, eliminating heat sinks and thereby significantly reducing overall system cost, size and weight. The class-D amplifiers work by converting an analog or digital input signal into a two-level output signal using a high-frequency modulation process. This two-level signal is then fed to a power stage to switch power switches in a full or half H-bridge, which in turn feeds a passive output filter connected to a speaker. These power switches have very low switching loss, so that the amplifier has better efficiency. Most class-D amplifier systems employ a pulse width modulation (PWM) scheme, where the value of the input signal at a moment in time is represented by a fixed-voltage variable-width output pulse. A typical audio PWM amplifier works at a switching frequency of between 100 KHz and 500 KHz. Higher switching frequencies can reduce distortion but also result in lower efficiency due to the extra transitions in the output waveform. However as is well known, the class-D amplifiers have electromagnetic interference (EMI) problem, as do the majority of switching power management devices, since they are derived from switching power supply configuration.
In further detail, referring to FIG. 1, a conventional class-D amplifier in bridge-tied-load (BTL) configuration always generates two PWM signals Out+ and Out− with 180 degrees out of phase at a pair of differential outputs, and produces the differential output (Out+)−(Out−) which has an amplitude (2Vdd) twice as large as that of the PWM signal Out+ or Out−. This large amplitude results in large ripple of the load current and therefore, there is always need for an external filter to remove the high frequency switching carrier. This filter typically includes at least two high current inductors and three capacitors which are expensive and consume undesirable amounts of space. In addition, the LC filter can also ease the EMI problem since the 180 degrees out of phases PWM signals Out+ and Out− generate large EMI interference. For this reason, filterless class-D amplifiers are proposed, for example by U.S. Pat. Nos. 6,614,297, 6,847,257 and 6,970,123, which eliminate the output filter while reducing the EMI interference.
FIG. 2 is a timing diagram to illustrate the operation of a general differential input class-D amplifier, in which two differential input signals Vin+ and Vin− are compared with a sawtooth carrier Vref, respectively, to generate two PWM signals PWM+ and PWM− whose subtraction is the output PWM signal to drive the power stage of the class-D amplifier. This output PWM signal has the same amplitude as those of the PWM signals PWM+ and PWM−, and thus the load current will have smaller ripple.
For a differential input class-D amplifier, the input information and the output power are designed in the difference of the signals PWM+ and PWM−. It is obvious that the pulse difference by the signals PWM+ or PWM− compared to 50% duty cycle is half of the output PWM signal, as shown in FIG. 2. In other words, the output PWM is symmetric to the 50% duty cycle. The output power of a single-end input class-D amplifier would be a quarter times less than that of which uses fully differential signals as inputs under a same gain. Conventionally, the single-end input signal is always processed by a gain stage which turns the single-end input signal into a fully differential input signal. The subsequent ternary or quaternary PWM control signals would need to be generated by using the fully differential input signal, or under a similar idea, using two mutually 180 degrees out of phase reference waveforms of sawtooth or triangle to generate the ternary or quaternary PWM control signals.
A new modulation scheme for class-D amplifiers is proposed under the concept illustrated in FIG. 2, which can keep the output power as large as that by using differential input signals in prior arts although the input signal is single-end and the entire signal processing path is kept single-ended.